The present invention relates to a tire for vehicle wheels comprising: a carcass structure having at least one carcass ply formed of at least one thread element disposed along deposition paths each extending in a substantially U-shaped conformation about the profile in transverse section of the tire, and a pair of circumferentially inextensible annular structures each engaged close to a respective circumferential inner edge of the carcass ply; a belt structure applied to the carcass structure at a circumferentially external position thereof; a tread band applied to the belt structure at a circumferential external position thereof; at least one pair of sidewalls applied to the carcass structure at laterally opposite positions.
Manufacture of tires for vehicle wheels involves formation of a carcass structure essentially consisting of one or more carcass plies substantially having a toroidal conformation and the axially opposite side edges of which engage respective circumferentially inextensible annular reinforcing elements called "bead cores".
Applied to the carcass structure, at a circumferentially external position thereof, is a belt structure comprising one or more belt strips in the form of a closed ring, essentially consisting of textile or metal cords suitably oriented relative to each other and to the cords belonging to the underlying carcass plies.
A tread band currently consisting of a strip of elastomer material of appropriate thickness is applied to the belt structure, at a circumferentially external position thereof. It is to point out that, to the aims of the present invention, by the term "elastomer material" it is intended a rubber blend in its entirety, that is the assembly made up of a base polymer suitably amalgamated with mineral fillers and/or additives of any other type.
Finally, to the opposite sides of the tire being manufactured a pair of sidewalls are applied, each of them covering a side portion of the tire included between a so-called shoulder region, located close to the corresponding side edge of the tread band, and a so-called bead located at the corresponding bead core.
In accordance with traditional production methods, essentially the above-listed tire components are first made separately from each other to be then assembled during a tire-manufacturing step.
For instance, for making the carcass ply or plies to be associated with the bead cores to form the carcass structure, production of a rubberized fabric comprising longitudinally-disposed continuous textile or metallic cords, through an extrusion and/or calendering process, is first required. This rubberized fabric is submitted to a transverse-cutting operation to produce lengths of predetermined sizes that are subsequently joined together so as to give rise to a continuous semifinished product having transversely-disposed parallel cords.
Then this article of manufacture is to be cut into pieces the length of which is correlated with the circumferential extension of the carcass to be made.
Production methods have been recently proposed which, instead of resorting to the production of semifinished products, make the carcass structure directly during the tire-manufacturing step.
For example, U.S. Pat. No. 5,453,140 herein quoted as an example of the most pertinent state of the art, discloses a method and an apparatus according to which the carcass structure is formed by laying down a single continuous thread according to alternating deposition paths placed consecutively in side-by-side relationship in a circumferential direction, onto a toroidal support having a shape corresponding to the inner shape of the tire to be made.
In more detail, the toroidal support is previously coated with a raw-rubber layer having a dual function, i.e., that of conveniently adhering to the deposited thread so as to hold the individual deposition sections thereof at a fixed positioning, and that of forming an air-proof inner liner in the finished tire.
The individual thread, directly drawn from a reel, is engaged by sliding members leading it to a movable guide member operating at the toroidal support. The movable guide member moves in a sliding path of travel having an advance section and a return section mutually interconnected to form an endless line lying in a plane radial to the toroidal support. Each of the advance and return sections extends in a substantially C-shaped configuration around the profile in transverse section of the toroidal support.
In this way, each time the guide element covers one of the advance or return sections of the sliding path of travel, deposition of the thread onto the toroidal support is caused, thereby forming a deposition section extending in a U-shaped conformation around the profile in transverse section of the toroidal support itself. At the instant intervening between formation of a deposition section and formation of the subsequent deposition section, the toroidal support is rotated through a predetermined angular pitch, making the apparatus ready for formation of a new deposition section disposed circumferentially in side-by-side relationship with the previously deposited section.
Pick-up devices making use of fork-shaped elements engage the thread at the end region of the just-formed deposition section, to prevent dragging along of the latter by the guide member during the initial formation step of the subsequent deposition section. Retention devices making use of presser elements conveniently act at the transition region between two deposition sections in succession to cause the end flaps thereof to adhere to the side surface of the toroidal support.
Tires obtained by this production method have a carcass structure in which the cords forming the carcass ply or plies consist of a single thread element forming a plurality of consecutive sections transverse to the tire, disposed parallelly in side-by-side relationship in a circumferential direction and deposited in respectively opposite directions so as to define an alternated course.
Within the scope of the carcass structure manufacture, as can be learned from Patents EP 0 664 231 and EP 0 664 232, the deposition sections formed by the individual thread element are also provided to be placed in an, alternated sequence at axially opposite positions relative to one or more annular anchoring elements constituting said bead cores.
In accordance with the present invention, different advantages have been found to be achieved if the carcass ply or plies are made by depositing at least one strip element essentially comprising a layer of raw elastomer material incorporating two or more parallel thread elements disposed longitudinally, in alternated consecutive sections transverse to the tire.